Caffeine and stress are known to have similar effects on the cardiovascular and neuroendocrine systems of the body. This similarity suggests that caffeine consumption will intensify the cardiovascular and neuroendocrine hyperresponsivity to stress which is thought to have pathological effects leading to cardiovascular disease. Three proposed studies will examine whether caffeine does intensify cardiovascular responses to stress. In the first, caffeine and placebo will be administered double-blind in two sessions with counter-balanced order to subjects who do not consume caffeine regularly. Measurements of heart rate, blood pressure, forearm blood flow, respiration, and electrodermal activity will be recorded during periods of rest and during the performance of a stressful mental arithmetic task. Compared to placebo, caffeine is expected to affect resting levels of the cardiovascular measures and to increase the responses elicited by the stressful task. Gender and time-of-day will also be examined as independent factors in this study. The second study will examine the interaction of caffeine and stress in regular coffee drinkers who are deprived overnight. Physiological measurements will again be recorded under resting and stress conditions. This study will provide information regarding the habituation of caffeine and stress interactions during laboratory stressors. A third study will examine the effects of caffeine during normal daily activities. Heart rate and blood pressure will be monitored in regular coffee drinkers who are engaged in their normal work activities. Caffeine and placebo will each be administered double-blind on several work-days matched for level of activity and stress. Order will be counter-balanced. Ambulatory measures of heart rate and blood pressure will be taken at 20-min. intevals from 9 A.M. until 1 P.M. and an activity and stress log will be kept. It is hypothesized that the average blood pressure and heart rate will be higher on caffeine days than placebo days due to an interaction of caffeine administration with the effects of everyday stress. In all studies, individual differences in sensitivity to caffeine and stress will be examined as will the influence of the Type A, coronary-prone behavior pattern. Demonstration that caffeine can intensify the potentially harmful effects of stress on the cardiovascular system will have important implications for the prevention and management of heart disease and hypertension.